first you want it to have a impish nature. that cuts speed for defense
second for it's ability iron barbs. that's it signature ability
third have it hold the rocky helmet that with iron barbs is sure to annoying those physical types
forth EVs in defense and sph defense
and finally it's moves
first have it know toxic the TM is in the sea side cave
second curse cuts speed (like it can out speed anything) for attack and defense
third payback is always fun if it moves last in battle it has a power of 100 rather than 50
and forth explosion to deal a lot of pain for when your health is in the red

now just some final things this works for my play style and by mo means am i competitive my online play record is as good as glass Joe's i just like to have a little fun and if your like me it will work fine

An interesting Pokemon, I expect to see it a lot. Have it hold that new helmet item (Rocky Helmet), and attacking it will knock away 25% of the foes total health. Plus, it has the ever popular Stealth Rock. Bug/Steel is better typing in my opinion.

Ability: Iron Barbs

EVs: Defense, Sp Def, Attack Nature: Adament

Explosion: About to be defeated? Kill everything in sight!

Leech Seed: Damage the foe even more.

Toxic: More damage to the foe. Did you know that in Gen1, Toxic made Leech Seed heal you more?

Stealth Rock/Seed Bomb: Okay, I hate using entry hazards, but they are really popular. If you are like me though, go for Grass STAB.

This guy is a pain in the rear end. First you Paralyze them with Thunder-wave to cut their speed and chances off hitting you. This gives you a chance to set up Substitute to block damage and Leech-seed for healing and damaging. Power-whip is STAB. Between Rocky-helmet and Iron-barbs any Foe that tries to physically hit Ferrothorn will lose tons of HP, not to mention damage from Leech-seed.

You could also go fast sub-seed if you really want to.

Just swap T-wave for Rock-polish and/or give it Jolly nature and put some EVs in Speed. Also if you give it Jolly and T-wave you may want to swap Power-whip for Iron-head for some annoying Para-flinch.

This version is meant to be a setup for a sweepers. The idea is pulling off both Stealth Rock and Spikes and do a lot of damage after doing so.

Ferrothorn @ Rocky Helmet
Ability: Iron Barbs
Nature: Impish/Brave
EV's: Def and Atk
-Ingrain/ Leach seed- I prefer ingrain cuz of its field presence
-Spikes
-Stealth rock
-Explosion/Power whip/Gyroball-
Explosion is just for the fast damage, PW will help against a lot of the rapid spinners (except forretress) and Gyro Ball is a good STAB.

Iron Head -Gyro ball sucks. It's unpredictable and max PP is 8. Iron head works better for the adventuring type, challenge to elite four, ect.-

Ingrain -Leech seed has to be breed in, AND you have to renew it every time the foe switches or dies.-

Power whip -Seems reliable enough, again you don't have to breed it-

Anything from Explosion, to Toxic, to Sandstorm, to an entry hazard. I find this one tough to decide on. Curse is nice, but it's too much geared toward gyro ball for my tastes.

Leftovers or Rocky helmet, one keeps you alive, the others kills the enemy. I'm a bit more conservative about this, so leftovers work for me -that plus ingrain is a pretty good hp intake every turn. Rocky helmet only works if you get attacked by a physical move-.

Any nature that lowers Speed or Spl Attack -except for the nature that affects just those two- is great for setting up a big wall of sorts. Of course maybe if you did give up Speed for Spl Attack, you can vary your attacks and use maybe Solar Beam or Flash Cannon.

Ferrothorn leftovers
Trait Iron Barbs
252HP 128defense 128SpD careful
Leech seed/ingrain nice healing with leftovers either way but with ingrain you can't switch out I personally prefer leech seed which can heal the team
Stealth rock/spikes entry hazard
Toxic/thunder wave thunder wave is nice if your other Pokemon aren't real fast. And toxic as a reliable damage on a tank or something.
Gyro ball/ power whip/iron head which ever one you choose. I prefer power whip with thunder wave but gyro ball if you chose toxic or you could do iron head and thunder wave for a little not very likely para flinch . It won't work amazingly well though.

This build is a Combo between a Physical Wall and a Physical Sweeper. By using curse your Defense and Offense go up as your Speed goes down. Gyro Ball, the slower the user, the greater the damage. As you gain power and prepare you not only can take physical hits, you bounce alot of damage back with the Rocky Helmet / Iron Barbs combo.